fix(doctor): improve AST-Grep NAPI detection for bunx environments

Use dynamic import instead of require.resolve() to detect @ast-grep/napi
installation. This fixes false negatives when running via bunx where the
module exists in ~/.config/opencode/node_modules but isn't resolvable
from the temporary execution directory.

Also adds fallback path checks for common installation locations.

Fixes #898

src/cli/doctor/checks/dependencies.test.ts

@@ -16,10 +16,10 @@ describe("dependencies check", () => {
   })
 
   describe("checkAstGrepNapi", () => {
-    it("returns dependency info", () => {
+    it("returns dependency info", async () => {
       // #given
       // #when checking ast-grep napi
-      const info = deps.checkAstGrepNapi()
+      const info = await deps.checkAstGrepNapi()
 
       // #then should return valid info
       expect(info.name).toBe("AST-Grep NAPI")
@@ -95,7 +95,7 @@ describe("dependencies check", () => {
 
     it("returns pass when installed", async () => {
       // #given napi installed
-      checkSpy = spyOn(deps, "checkAstGrepNapi").mockReturnValue({
+      checkSpy = spyOn(deps, "checkAstGrepNapi").mockResolvedValue({
         name: "AST-Grep NAPI",
         required: false,
         installed: true,